public class test {
    public static int zhaoNumber(int[] arr){
        int mid = 0;
        for (int i = 0; i < arr.length; i++) {
            mid ^= arr[i];
        }
        return mid;
    }
    public static void main(String[] args) {
        int[] arr = {1,1,2,2,3,3,4,4,5,5,6,6,7};
        int n = zhaoNumber(arr);
        System.out.println(n);
    }
}
